Add databriks structure to field when it is null and not null in some rows.

addStructure.py

def addStructure(df, field, structFields):
    '''
    Receive a df, a field and structure in a dictionary. 
    '''
    # compruebas si todos los valores son nulos, pq cuando lo son no funciona el replace de null por una estructura. 
    validator = df.select(df[field]).distinct()
    if validator.schema[field].dataType.typeName() == 'string':
        otherwiseField =  None
    else:
        otherwiseField =  df[field]
    
    # crear la estructura qeur deberían tener esos campos nulos. 
    for k in structFields:
        listStruct = [f.lit('').alias(item) for item in structFields]
    jsonFields = f.struct(*listStruct)
    
    df = df.withColumn(field, f.when(df[field].isNull(), jsonFields).otherwise(otherwiseField)
    return df
 '''
 Use case:
   * have a dataframe with employee fields as structure:
       - struct(Name: string, surname: string)
   * we want to flatten this fields, and dont' have nulls or almost have the columns created when all rows are null for field employee
   
   df = addStructure(df, 'employee', ['name', 'surname'])
    
   return df